*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }

:root {
    --bg: #1a1e24;
    --surface: #22272e;
    --surface2: #2d333b;
    --border: #3d444d;
    --text: #e6edf3;
    --text-dim: #8b949e;
    --accent: #58a6ff;
    --accent-hover: #79c0ff;
    --red: #f85149;
    --green: #3fb950;
    --orange: #d29922;
    --input-bg: #161b22;
    --radius: 8px;
    --log-export-bg: #1a1e24;
    --log-export-text: #e6edf3;
    --log-export-rule: #3d444d;
}

[data-theme="light"] {
    --bg: #eef1f5;
    --surface: #ffffff;
    --surface2: #f6f8fa;
    --border: #d0d7de;
    --text: #1f2328;
    --text-dim: #656d76;
    --accent: #0969da;
    --accent-hover: #0550ae;
    --red: #cf222e;
    --green: #1a7f37;
    --orange: #9a6700;
    --input-bg: #ffffff;
    --log-export-bg: #ffffff;
    --log-export-text: #1f2328;
    --log-export-rule: #d0d7de;
}

body {
    font-family: 'Segoe UI', -apple-system, BlinkMacSystemFont, sans-serif;
    background: var(--bg);
    color: var(--text);
    line-height: 1.5;
    min-height: 100vh;
}

/* ── Header ── */

header {
    background: var(--surface);
    border-bottom: 1px solid var(--border);
    padding: 16px 32px;
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 16px;
}

.header-main {
    display: flex;
    align-items: center;
    gap: 16px;
    flex-wrap: wrap;
    min-width: 0;
}

header h1 {
    font-size: 1.25rem;
    font-weight: 600;
}

header .badge {
    background: var(--accent);
    color: #000;
    font-size: 0.7rem;
    font-weight: 700;
    padding: 2px 8px;
    border-radius: 12px;
    text-transform: uppercase;
    letter-spacing: 0.5px;
    flex-shrink: 0;
}

.btn-help {
    width: 40px;
    height: 40px;
    border-radius: 50%;
    border: 1px solid var(--border);
    background: var(--surface2);
    color: var(--accent);
    font-size: 1.15rem;
    font-weight: 700;
    font-family: inherit;
    line-height: 1;
    cursor: pointer;
    flex-shrink: 0;
    transition: background 0.2s, color 0.2s, border-color 0.2s, box-shadow 0.2s;
}

.btn-help:hover {
    background: var(--accent);
    color: #0d1117;
    border-color: var(--accent);
    box-shadow: 0 0 0 3px rgba(88, 166, 255, 0.25);
}

.btn-help:focus-visible {
    outline: 2px solid var(--accent);
    outline-offset: 2px;
}

.header-toolbar {
    display: flex;
    align-items: center;
    gap: 10px;
    flex-shrink: 0;
}

.lang-switcher {
    display: flex;
    border: 1px solid var(--border);
    border-radius: 8px;
    overflow: hidden;
    background: var(--surface2);
}

.lang-btn {
    padding: 6px 10px;
    border: none;
    background: transparent;
    color: var(--text-dim);
    font-size: 0.72rem;
    font-weight: 700;
    font-family: inherit;
    cursor: pointer;
    letter-spacing: 0.04em;
    transition: background 0.15s, color 0.15s;
}

.lang-btn:hover {
    color: var(--text);
    background: var(--border);
}

.lang-btn--active {
    background: var(--accent);
    color: #fff;
}

[data-theme="light"] .lang-btn--active {
    color: #fff;
}

.lang-btn + .lang-btn {
    border-left: 1px solid var(--border);
}

.btn-theme {
    width: 40px;
    height: 40px;
    border-radius: 50%;
    border: 1px solid var(--border);
    background: var(--surface2);
    color: var(--accent);
    font-size: 1.1rem;
    line-height: 1;
    cursor: pointer;
    flex-shrink: 0;
    display: flex;
    align-items: center;
    justify-content: center;
    transition: background 0.2s, border-color 0.2s, box-shadow 0.2s;
}

.btn-theme:hover {
    border-color: var(--accent);
    box-shadow: 0 0 0 3px rgba(88, 166, 255, 0.2);
}

[data-theme="light"] .btn-theme:hover {
    box-shadow: 0 0 0 3px rgba(9, 105, 218, 0.2);
}

.btn-theme:focus-visible {
    outline: 2px solid var(--accent);
    outline-offset: 2px;
}

body.modal-open {
    overflow: hidden;
}

.modal-overlay {
    position: fixed;
    inset: 0;
    z-index: 200;
    display: flex;
    align-items: center;
    justify-content: center;
    padding: 24px;
    background: rgba(0, 0, 0, 0.65);
    backdrop-filter: blur(4px);
}

.modal-overlay[hidden] {
    display: none;
}

.modal-panel {
    width: 100%;
    max-width: 720px;
    max-height: min(85vh, 720px);
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: 12px;
    box-shadow: 0 24px 64px rgba(0, 0, 0, 0.5);
    display: flex;
    flex-direction: column;
    overflow: hidden;
}

.modal-header {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 16px;
    padding: 20px 24px;
    border-bottom: 1px solid var(--border);
    background: var(--surface2);
    flex-shrink: 0;
}

.modal-header h2 {
    font-size: 1.1rem;
    font-weight: 600;
    margin: 0;
}

.modal-close {
    width: 36px;
    height: 36px;
    border: none;
    border-radius: 8px;
    background: transparent;
    color: var(--text-dim);
    font-size: 1.5rem;
    line-height: 1;
    cursor: pointer;
    flex-shrink: 0;
    transition: background 0.15s, color 0.15s;
}

.modal-close:hover {
    background: var(--border);
    color: var(--text);
}

.modal-body {
    padding: 20px 24px 24px;
    overflow-y: auto;
    font-size: 0.9rem;
    color: var(--text);
    line-height: 1.6;
}

.modal-section {
    margin-bottom: 24px;
}

.modal-section:last-child {
    margin-bottom: 0;
}

.modal-section h3 {
    font-size: 0.8rem;
    font-weight: 700;
    text-transform: uppercase;
    letter-spacing: 0.06em;
    color: var(--accent);
    margin: 0 0 10px;
}

.modal-section p {
    margin: 0 0 10px;
    color: var(--text);
}

.modal-section ul {
    margin: 0 0 10px;
    padding-left: 1.25rem;
    color: var(--text-dim);
}

.modal-section li {
    margin-bottom: 6px;
}

.modal-section li strong {
    color: var(--text);
}

.modal-section a {
    color: var(--accent);
    text-decoration: none;
}

.modal-section a:hover {
    text-decoration: underline;
}

.modal-disclaimer {
    margin-top: 12px !important;
    padding: 12px 14px;
    border-left: 3px solid var(--orange);
    background: rgba(210, 153, 34, 0.08);
    border-radius: 0 8px 8px 0;
    font-size: 0.85rem;
    color: var(--text-dim) !important;
}

.modal-figure {
    margin: 0 0 16px;
    border: 1px solid var(--border);
    border-radius: 8px;
    overflow: hidden;
    background: #f6f8fa;
}

.modal-figure img {
    display: block;
    width: 100%;
    height: auto;
    vertical-align: middle;
}

.modal-figure figcaption {
    padding: 12px 14px;
    font-size: 0.8rem;
    line-height: 1.55;
    color: var(--text-dim);
    background: var(--surface2);
    border-top: 1px solid var(--border);
}

.app-footer {
    text-align: center;
    padding: 16px 24px 24px;
    font-size: 0.75rem;
    color: var(--text-dim);
    border-top: 1px solid var(--border);
    margin-top: 8px;
}

/* ── Layout ── */

.app {
    max-width: 1400px;
    margin: 0 auto;
    padding: 24px;
    display: grid;
    grid-template-columns: 1fr 1fr;
    grid-template-rows: auto auto auto;
    gap: 20px;
}

.input-section  { grid-column: 1; grid-row: 1 / 4; }
.output-section { grid-column: 2; grid-row: 1; }
.parallel-section { grid-column: 2; grid-row: 2; }
.log-section    { grid-column: 2; grid-row: 3; }

/* ── Cards ── */

.card {
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 20px;
}

.card-title {
    font-size: 0.8rem;
    font-weight: 600;
    text-transform: uppercase;
    letter-spacing: 1px;
    color: var(--text-dim);
    margin-bottom: 16px;
    padding-bottom: 8px;
    border-bottom: 1px solid var(--border);
}

/* ── Form ── */

.form-group {
    margin-bottom: 20px;
}

.form-group-title {
    font-size: 0.85rem;
    font-weight: 600;
    color: var(--accent);
    margin-bottom: 12px;
    display: flex;
    align-items: center;
    gap: 8px;
}

.form-group-title .icon {
    width: 20px;
    height: 20px;
    display: inline-flex;
    align-items: center;
    justify-content: center;
    font-size: 0.75rem;
}

.field-row {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 12px;
    margin-bottom: 8px;
}

.field {
    display: flex;
    flex-direction: column;
    gap: 4px;
}

.field.full-width {
    grid-column: 1 / -1;
}

.field-note {
    margin: 0;
    font-size: 0.75rem;
    color: var(--text-dim);
    line-height: 1.55;
    max-width: 52rem;
}

.field-note--warn {
    border-left: 3px solid var(--orange);
    padding-left: 12px;
    margin-top: 10px;
}

label {
    font-size: 0.8rem;
    color: var(--text-dim);
    font-weight: 500;
}

label .unit {
    color: var(--accent);
    font-weight: 400;
}

/* ── Inputs ── */

input, select {
    background: var(--input-bg);
    border: 1px solid var(--border);
    border-radius: 6px;
    color: var(--text);
    padding: 8px 12px;
    font-size: 0.9rem;
    font-family: inherit;
    transition: border-color 0.15s, box-shadow 0.15s;
    width: 100%;
}

input:focus, select:focus {
    outline: none;
    border-color: var(--accent);
    box-shadow: 0 0 0 3px rgba(88, 166, 255, 0.15);
}

input.invalid {
    border-color: var(--red);
    box-shadow: 0 0 0 3px rgba(248, 81, 73, 0.15);
}

select {
    cursor: pointer;
    appearance: none;
    background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='12' height='12' fill='%238b949e' viewBox='0 0 16 16'%3E%3Cpath d='M8 11L3 6h10z'/%3E%3C/svg%3E");
    background-repeat: no-repeat;
    background-position: right 10px center;
    padding-right: 32px;
}

/* ── Buttons ── */

.btn {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    gap: 8px;
    padding: 12px 24px;
    border: none;
    border-radius: 6px;
    font-size: 0.95rem;
    font-weight: 600;
    font-family: inherit;
    cursor: pointer;
    transition: background 0.15s, transform 0.1s, opacity 0.15s;
}

.btn:active { transform: scale(0.98); }

.btn-primary {
    background: var(--accent);
    color: #000;
    width: 100%;
    margin-top: 8px;
}

.btn-primary:hover { background: var(--accent-hover); }
.btn-primary:disabled { opacity: 0.4; cursor: not-allowed; }

.btn-secondary {
    background: var(--surface2);
    color: var(--text);
    border: 1px solid var(--border);
}

.btn-secondary:hover { background: var(--border); }
.btn-secondary:disabled { opacity: 0.4; cursor: not-allowed; }

/* ── Output ── */

.formula-results {
    margin-top: 16px;
}

.formula-results .output-grid {
    margin-bottom: 16px;
}

.formula-results .info-row {
    margin-bottom: 0;
}

.output-grid {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 16px;
    margin-bottom: 20px;
}

.result-card {
    background: var(--input-bg);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 16px;
    text-align: center;
}

.result-card .result-label {
    font-size: 0.75rem;
    color: var(--text-dim);
    text-transform: uppercase;
    letter-spacing: 0.5px;
    margin-bottom: 4px;
}

.result-card .result-value {
    font-size: 1.8rem;
    font-weight: 700;
    color: var(--red);
    font-variant-numeric: tabular-nums;
}

.result-card .result-unit {
    font-size: 0.85rem;
    color: var(--text-dim);
    font-weight: 400;
}

/* ── Formula ── */

.formula-box {
    background: var(--input-bg);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 16px;
    text-align: center;
    margin-bottom: 16px;
    transition: border-color 0.2s, box-shadow 0.2s, opacity 0.2s;
}

.formula-box:not(.formula-box--inactive) {
    border-color: rgba(88, 166, 255, 0.35);
    box-shadow: 0 0 0 1px rgba(88, 166, 255, 0.08), 0 4px 16px rgba(0, 0, 0, 0.2);
}

.formula-box .formula-title {
    font-size: 0.75rem;
    color: var(--accent);
    text-transform: uppercase;
    letter-spacing: 0.5px;
    margin-bottom: 8px;
}

.formula-box .formula {
    font-size: 1.1rem;
    font-family: 'Cambria Math', 'Latin Modern Math', Georgia, serif;
    color: var(--text);
    line-height: 2.2;
}

.formula .frac {
    display: inline-flex;
    flex-direction: column;
    align-items: center;
    vertical-align: middle;
    margin: 0 4px;
}

.formula .frac .num { border-bottom: 1.5px solid var(--text-dim); padding: 0 6px 2px; }
.formula .frac .den { padding: 2px 6px 0; }

.formula .formula-tail {
    margin-left: 2px;
    white-space: nowrap;
}

.formula-legend {
    font-size: 0.78rem;
    color: var(--text-dim);
    margin-top: 12px;
    padding-top: 12px;
    border-top: 1px solid var(--border);
    text-align: left;
    line-height: 1.8;
}

.formula-legend span {
    color: var(--accent);
    font-weight: 600;
}

.formula-box .formula-title .formula-regime {
    font-weight: 400;
    text-transform: none;
    letter-spacing: 0;
    opacity: 0.85;
}

#pressureRegimeBanner {
    margin-bottom: 14px;
    font-size: 0.82rem;
    color: var(--accent);
}

.formula-box--inactive {
    opacity: 0.38;
    pointer-events: none;
    filter: grayscale(0.4);
    border-color: var(--border);
    box-shadow: none;
}

.formula-box--inactive .formula-title {
    color: var(--text-dim);
}

.formula-box--inactive .formula-legend span {
    color: var(--text-dim);
    font-weight: 500;
}

.formula.form--ch9 {
    line-height: 1.9;
}

.formula-ch9-core {
    display: inline-block;
    margin-left: 4px;
    padding: 4px 10px;
    border-radius: 6px;
    background: rgba(88, 166, 255, 0.06);
    border: 1px solid rgba(88, 166, 255, 0.15);
}

.formula--ch9 .formula-ch9-core {
    color: var(--text);
}

.parallel-section .formula-box {
    margin-bottom: 0;
}

.formulas-dual {
    display: flex;
    flex-direction: column;
    gap: 14px;
}

#a1StressDisplay,
#k1StressDisplay {
    margin-top: 6px;
    font-size: 0.82rem;
    color: var(--accent);
}

#a1StressDisplay[hidden],
#k1StressDisplay[hidden] {
    display: none !important;
}

.parallel-section .btn {
    margin-top: 6px;
    margin-bottom: 14px;
}

.parallel-ch9-log {
    margin-top: 12px;
    min-height: 72px;
    font-size: 0.78rem;
}

/* ── Log ── */

.log-container {
    background: var(--input-bg);
    border: 1px solid var(--border);
    border-radius: 6px;
    padding: 12px 16px;
    min-height: 200px;
    max-height: 400px;
    overflow-y: auto;
    font-family: 'Consolas', 'Cascadia Code', monospace;
    font-size: 0.8rem;
    line-height: 1.7;
    color: var(--text-dim);
    margin-bottom: 12px;
    white-space: pre-wrap;
}

.log-container:empty::before {
    content: "Calculation log will appear here after running...";
    color: var(--border);
    font-style: italic;
}

.log-actions {
    display: flex;
    flex-wrap: wrap;
    gap: 8px;
}

/* ── Misc ── */

.separator {
    height: 1px;
    background: var(--border);
    margin: 16px 0;
}

.info-row {
    display: flex;
    align-items: center;
    gap: 8px;
    padding: 8px 12px;
    background: rgba(88, 166, 255, 0.08);
    border: 1px solid rgba(88, 166, 255, 0.2);
    border-radius: 6px;
    font-size: 0.8rem;
    color: var(--accent);
    margin-bottom: 16px;
}

/* ── THK alert ── */

.alert-thk {
    display: flex;
    align-items: flex-start;
    gap: 10px;
    margin-top: 12px;
    padding: 12px 16px;
    border-radius: 8px;
    background: rgba(255, 70, 70, 0.12);
    border: 1px solid var(--red, #f44);
    color: #ff6b6b;
    font-size: 0.85rem;
    font-weight: 600;
    line-height: 1.4;
}

.alert-thk[hidden] { display: none; }

.alert-thk__icon {
    font-size: 1.2rem;
    flex-shrink: 0;
}

.toast {
    position: fixed;
    bottom: 24px;
    right: 24px;
    background: var(--green);
    color: #000;
    padding: 12px 20px;
    border-radius: 8px;
    font-weight: 600;
    font-size: 0.85rem;
    transform: translateY(80px);
    opacity: 0;
    transition: transform 0.3s, opacity 0.3s;
    z-index: 100;
}

.toast.show { transform: translateY(0); opacity: 1; }

/* ── Responsive ── */

@media (max-width: 900px) {
    .app {
        grid-template-columns: 1fr;
    }
    .input-section  { grid-column: 1; grid-row: 1; }
    .output-section { grid-column: 1; grid-row: 2; }
    .parallel-section { grid-column: 1; grid-row: 3; }
    .log-section    { grid-column: 1; grid-row: 4; }
}